1. Field of the Invention
The present invention is related to a system for providing increased data speed using high resolution analog to digital conversion for use in communication systems. In particular, the present invention relates to method and apparatus for increasing pulse amplitude modulated (PAM) data transmission speed in communications systems such as pair gain systems by increasing the bandwidth allocated to each subscriber.
2. Description of the Related Art
The presently available 56K modems achieve their high transmission speeds by training the analog to digital (A/D) conversion process which takes place in the transmission path between the central office unit and the remote terminal unit, or between the customer site unit and the Internet Service Provider (ISP). The higher resolution gained by the A/D converter is maintained at the terminal end of the pair gain system by a D/A converter of the same resolution.
However, the transmission speed obtained by the 56K modems is significantly reduced when an additional A/D conversion process is introduced into the data transmission path. This degradation in speed is caused by the quantization noise of the additional A/D conversion. The quantization noise of an A/D converter is a function of the resolution of the A/D conversion process. In turn, the resolution of the A/D conversion process is governed by the number of bits associated with each sample. The A/D converter used in typical telephony systems is an 8-bit companded converter, companding referring to non-linear conversion characteristics of the A/D converter.
In a pair gain system, the additional A/D converter added in the transmission path is under the control of the pair gain system supplier. Moreover, the A/D converter in a pair gain system does not need to be an 8-bit companded type as generally used in the typical telephony systems. In fact, the A/D converter in a pair gain system can have a greater number of bits associated with each sample than the typical 8 bits. These extra bits tend to decrease the quantization noise and increase the attainable modem data transmission rates. However, since a greater number of bits must be carried over the pair gain system, the number of available subscribers in the pair gain system is reduced.
FIG. 1 illustrates an eight line copper fed subscriber carrier system available from GoDigital Networks Corp. of Fremont Calif., the assignee of the present invention, a detailed description of which is provided in an application entitled Multiple Digital Subscriber Carrier With Drop and Insert Repeater System concurrently filed herein which claims priority under 35 USC xc2xa7119 to provisional application No. 60/121,011 filed on Feb. 22, 1999, the disclosures of each of which are incorporated in its entirety herein by reference.
As shown, Central Office Unit 101 is provided with eight Subscriber Line Access Circuits (SLACx) 104, each of which is configured to emulate telephone line termination. Also shown in FIG. 1 is Customer Site Unit 102 which, in this embodiment, is provided with eight Subscriber Line Interface Circuits (SLICx) 109, each of which is coupled to a subscriber line and configured to emulate the Central Office. The Central Office Unit 101 is coupled to the Customer Site Unit 102 via a single, bi-directional twisted copper cable pair 103. As further shown in FIG. 1, the Central Office Unit 101 and the Customer Site Unit 102 are each provided with eight 8-bit codecs 105, 108 each of which are respectively coupled to corresponding the SLAC 104 in the Central Office Unit 101, and to the corresponding SLIC 109 in the Customer Site Unit 102. Further provided are bi-directional data transmission buses which carry digital data bits from each of the 8-bit codecs 105, 108 in the Central Office Unit 101 and the Customer Site Unit 102, respectively, to control units such as framing and transport units 106, 107 which are coupled to the single twisted pair 103.
The 8-bit codecs 104 in the Central Office Unit 101 are configured to encode the analog data received from the Central Office switch (not shown) into a corresponding digital bit stream format, while the 8-bit codecs 108 in the Customer Site Unit 102 are configured to decode the digital bit stream received from the central Office Unit 101 via the single twisted cable pair 103 into the respective analog format. Accordingly, the analog signals received from the Central Office switch are transported to the remote site at the Customer Site Unit 102 such that the subscribers connected to the subscriber lines are able to receive the transmitted analog signals (and to transmit signals in the opposite direction to the Central Office Unit 101 via the same mechanism as described above). For example, a subscriber line may be connected to the Customer Site Unit 102 via a 56 Kb/s modem for signal transmission and reception.
FIG. 2 illustrates the framing and transport unit 106 in the 8-bit system of FIG. 1 in further detail. As shown, there is provided a framer 203 for assigning time slots to each 8-bit codecs in the Central Office Unit 101 and the Customer Site Unit 102, and also for controlling the signaling. By way of example, at the central office side, the signaling information may consist of off-hook, and ring detect while at the subscriber side, the signaling information may consist of ringing and off hook detection. This signaling information is added to the data bit stream such that the data rate at the transmission line consists of the codec data plus the signaling data. An additional 32 KHz may be allocated for signaling. With a given clock for the system, FIG. 2 shows, for example, timing chart for two time slots and the transmit data TX and receive data RX for the given time slots.
Further shown in FIG. 2 is a microprocessor 202 coupled to the framer 203 and transceiver 201 (for example, a DSP chip such as Brooktree BT 8960) for controlling and processing the operation of the framer 203 and the transceiver 201. The transceiver 201 is a transport chip configured for transmitting digital data stream from the 8-bit codecs 105 in the Central Office Unit 101 with the signaling data to a similar transceiver located at the Customer Site Unit 102 (FIG. 1) and vice versa.
In the subscriber carrier system shown in FIGS. 1 and 2, an extra A/D conversion process is required in the downstream path towards the subscribers in the Central Office Unit 101. If, for example, the subscriber is connected to the Customer Site Unit 102 via a 56 Kb/s modem, this extra A/D conversion required in the system described above reduces the subscriber modem speed by approximately 30% over the speed that could otherwise be achievable (without the additional A/D conversion process) such that the signal transmission rate is limited to 33.6 Kb/s. Therefore, it would be desirable to have a signal transmission system which minimizes the reduction of the signal transmission rate such that the subscribers are able to receive information at a higher data rate then currently available.
In view of the foregoing, there are provided method and apparatus for providing a system for increasing pulse amplitude modulated (PAM) data transmission speed in communication systems such as pair gain systems by increasing the bandwidth allocated to each subscriber. In particular, in accordance with the present invention, data transmission speed in communication systems are substantially increased by the higher resolution A/D and D/A conversion processes of the present invention to mimic the companded conversion process of the existing 8-bit telephony type codecs.
In accordance with one embodiment of the present invention, there is provided a high speed data transmission system, comprising: a data transmission line; a central office unit coupled to the data transmission line for receiving analog signals from a central office switch and for receiving digital signals from the data transmission line, the central office unit including: a plurality of subscriber line access circuits each configured to interface with a central office switch; a first plurality of codecs coupled to a respective one of the subscriber line access circuits, each of said first plurality of codecs configured to receive analog signals from the central office switch via the respective subscriber line access circuits and to convert the analog signals to corresponding digital signals; a first data bus coupled to each of the first plurality of codecs for transporting the digital signals; and a first controller coupled to the first data bus configured to receive the digital signals via the first data bus for transmission to the data transmission line; and a customer site unit coupled to the data transmission line for receiving analog signals from one or more subscriber terminals and for receiving digital signals from the transmission line, the customer site unit including: a plurality of subscriber line interface circuits each configured to interface with a one or more subscriber terminals; a second plurality of codecs coupled to a respective one of the plurality of subscriber line interface circuits to receive analog signals and to convert the analog signals to corresponding digital signals; a second data bus coupled to each of the second plurality of codecs for transporting the digital signals; and a second controller coupled to the second data bus configured to receive the digital signals from the second plurality of codecs via the data bus for transmission to the data transmission line.
In accordance with another embodiment of the present invention, there is provided a central office unit, comprising: a plurality of subscriber line access circuits each configured to interface with a central office switch; a plurality of codecs coupled to a respective one of the plurality of subscriber line access circuits, each of said codecs configured to receive analog signals from the central office switch via the respective subscriber line access circuits and to convert the analog signals to corresponding digital signals; a data bus coupled to each of the codecs for transporting the digital signals; and a controller coupled to the data bus configured to receive the digital signals via the data bus for transmission to a transmission line.
In accordance with yet another embodiment of the present invention, there is provided a customer site unit, comprising: a plurality of subscriber line interface circuits each configured to interface with a one or more subscriber terminals; a plurality of codecs coupled to a respective one of the plurality of subscriber line interface circuits to receive analog signals and to convert the analog signals to corresponding digital signals; a data bus coupled to each of the codecs for transporting the digital signals; and a controller coupled to the data bus configured to receive the digital signals from the plurality of codecs via the data bus for transmission to a transmission line.
In accordance with a further embodiment of the present invention, there is provided a high speed data transmission method, comprising the steps of: providing a data transmission line; installing a central office unit for connection to the data transmission line for receiving analog signals from a central office switch and for receiving digital signals from the data transmission line, the step of installing the central office unit including the steps of: configuring a plurality of subscriber line access circuits for interface with a central office switch; providing a first plurality of codecs coupled to a respective one of the subscriber line access circuits, each of said first plurality of codecs configured to receive analog signals from the central office switch via the respective subscriber line access circuits and to convert the analog signals to corresponding digital signals; providing a first data bus coupled to each of the first plurality of codecs for transporting the digital signals; and providing a first controller to the first data bus configured to receive the digital signals via the first data bus for transmission to the data transmission line; and installing a customer site unit for connection to the data transmission line for receiving analog signals from one or more subscriber terminals and for receiving digital signals from the transmission line, the step of installing the customer site unit including the steps of: configuring a plurality of subscriber line interface circuits for interface with a one or more subscriber terminals; providing a second plurality of codecs coupled to a respective one of the plurality of subscriber line interface circuits to receive analog signals and to convert the analog signals to corresponding digital signals; providing a second data bus coupled to each of the second plurality of codecs for transporting the digital signals; and providing a second controller coupled to the second data bus configured to receive the digital signals from the second plurality of codecs via the data bus for transmission to the data transmission line.
In accordance with still a further embodiment of the present invention, there is provided a method of installing a central office unit in a communication system, comprising the steps of: providing a plurality of subscriber line access circuits each configured to interface with a central office switch; providing a plurality of codecs coupled to a respective one of the plurality of subscriber line access circuits, each of said codecs configured to receive analog signals from the central office switch via the respective subscriber line access circuits and to convert the analog signals to corresponding digital signals; providing a data bus coupled to each of the codecs for transporting the digital signals; and providing a controller coupled to the data bus configured to receive the digital signals via the data bus for transmission to a transmission line.
In accordance with yet still a further embodiment of the present invention, there is provided a method of installing customer site unit in a communication system, comprising the steps of: providing a plurality of subscriber line interface circuits each configured to interface with a one or more subscriber terminals; providing a plurality of codecs coupled to a respective one of the plurality of subscriber line interface circuits to receive analog signals and to convert the analog signals to corresponding digital signals; providing a data bus coupled to each of the codecs for transporting the digital signals; and providing a controller coupled to the data bus configured to receive the digital signals from the plurality of codecs via the data bus for transmission to a transmission line.
These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.